In today's radio communications networks a number of different technologies are used, such as Long Term Evolution (LTE), LTE-Advanced, Wideband Code Division Multiple Access (WCDMA), Global System for Mobile communications/Enhanced Data rate for GSM Evolution (GSM/EDGE), Worldwide Interoperability for Microwave Access (WiMax), or Ultra Mobile Broadband (UMB), just to mention a few possible technologies. A radio communications network comprises radio base stations providing radio coverage over at least one respective geographical area forming a cell. User equipments (UE) are served in the cells by the respective radio base station and are communicating with respective radio base station. The user equipments transmit data over a radio interface to the radio base stations in uplink (UL) transmissions and the radio base stations transmit data to the user equipments in downlink (DL) transmissions.
The possibility of identifying a geographical location of a user equipment in the radio communications network has enabled a large variety of commercial and non-commercial services e.g. navigation assistance, social networking, location-aware advertising, emergency calls, etc. Different services may have different positioning accuracy requirements imposed by the positioning application. In addition, some regulatory requirements on the positioning accuracy for basic emergency services exist in some countries, e.g. 300 meters in Federal Communications Commission (FCC) Enhanced 9-1-1 in the United States.
In many environments, a user equipment's position may be accurately estimated by using positioning methods based on the Global Positioning System (GPS). Nowadays, radio communications networks also often have a possibility to assist user equipments in order to improve the user equipment's receiver sensitivity and GPS start-up performance, e.g. as Assisted-GPS (A-GPS) positioning do. GPS or A-GPS receivers may, however, not necessarily be available in all user equipments. Furthermore, GPS is known to often fail in indoor environments and urban canyons. A complementary terrestrial positioning method, called Observed Time Difference of Arrival (OTDOA), has therefore been standardized by 3rd Generation Partnership Project (3GPP). In addition to OTDOA, the LTE standard also specifies methods, procedures, and signaling support for Enhanced Cell ID (E-CID) and Assisted-Global Navigation Satellite System (A-GNSS) positioning. In the future, Uplink Time Difference of Arrival (UTDOA) may also be standardized for LTE, which is a real time locating technology that uses multilateration based on timing of received uplink signals. Multilateration is the process of locating an object by accurately computing the Time Difference Of Arrival (TDOA) of a signal emitted from that object to three or more receivers.
Three key network elements in an LTE positioning architecture are the Location Service (LCS) Client, the LCS target device and the LCS Server. The LCS Server is a physical or logical entity managing positioning for a LCS target device by collecting measurements and other location information, assisting the user equipment in measurements when necessary, and estimating the LCS target location. A LCS Client is a software and/or hardware entity that interacts with a LCS Server for the purpose of obtaining location information for one or more LCS target devices, i.e. the entities being positioned. LCS Clients may reside in the LCS target devices themselves. An LCS Client sends a request to LCS Server to obtain location information, and LCS Server processes and serves the received requests and sends the positioning result and optionally a velocity estimate to the LCS Client. A positioning request may be originated from the user equipment or any network node.
Position calculation may be conducted, for example, by a positioning node, e.g. LCS server, Enhanced Serving Mobile Location Centre (E-SMLC), Secure User Plane Location (SUPL) Location Platform (SLP) in LTE, or user equipment. The former approach corresponds to the UE-assisted positioning mode, whilst the latter corresponds to the UE-based positioning mode.
The OTDOA positioning method mentioned above makes use of the measured timing of downlink signals received from multiple radio base stations at the user equipment. The user equipment measures the timing of the received signals using positioning assistance data received from the positioning node, and the resulting measurements are used to locate the user equipment in relation to the neighbouring radio base stations. Thus, with OTDOA, the user equipment measures the timing differences for downlink reference signals received from multiple distinct locations. For each measured neighbour cell, the user equipment measures Reference Signal Time Difference (RSTD) which is the relative timing difference between neighbour cell and a reference cell. The estimate of the user equipment position is then found as the intersection of hyperbolas, which is a geometrical curve, corresponding to the measured RSTDs. At least three measurements from geographically dispersed radio base stations with a good geometry are needed to solve for two coordinates of the user equipment and the user equipment receiver's clock bias. In order to position the user equipment, precise knowledge of the transmitter locations and transmit timing offset is needed.
To enable positioning in LTE and facilitate a positioning measurement of a proper quality and for a sufficient number of distinct locations, new physical signals dedicated for positioning e.g. Positioning Reference Signals (PRS), have been introduced and low-interference positioning subframes have been specified in 3GPP.
PRSs are transmitted from one antenna port, e.g. antenna port R6, according to a pre-defined pattern. A frequency shift, which is a function of Physical Cell Identity (PCI), may be applied to the specified PRS patterns to generate orthogonal patterns and modeling the effective frequency reuse of six patterns, which makes it possible to significantly reduce neighbour cell interference on the measured PRS and thus improve a positioning measurement. Even though PRSs have been specifically designed for a positioning measurement and in general are characterized by better signal quality than other reference signals, the standard does not mandate using PRS. Other reference signals, e.g. Cell-specific Reference Signals (CRS) may also be used for a positioning measurement.
PRSs are transmitted in a predefined pattern such as pre-defined positioning subframes grouped by a number N of consecutive subframes of PRS, NPRS for short, i.e. one positioning occasion. Positioning occasions occur periodically with a certain periodicity of the number N of subframes, i.e. the time interval between two positioning occasions. The standardized periods are 160, 320, 640, and 1280 ms stated in 3GPP TS 36.211 section 6.10.4.3, and the number of consecutive subframes are 1, 2, 4, and 6 as defined in 3GPP TS 36.355 section 6.5.1.2. Today, the user equipment may provide erroneous positioning measurements since the positioning measurements may rely on wrong assumptions made by the user equipment which may also be inconsistent with the information at the positioning node providing the positioning assistance data. Further, inconsistency in the implementation of the positioning parameters calculation with the network assumption on how the user equipment will do this may also lead to different interpretation of the parameters in the positioning assistance data by both sides and thus erroneous measurements in the end. The required positioning accuracy in a real network may thus not be ensured and the user equipment may not fulfill the positioning measurement requirements. In some systems this may be solved by a higher complexity in the user equipment e.g. the user equipment searches reference signals over a longer time to detect the reference signals. However, this will require more memory and more time and power for processing as well as this will likely to also violate positioning requirements such as measurement accuracy or measurement reporting delay requirements.